Activity hub

### Describe the problem

User notifications are currently centered around e-mail. This heavily limits what we can do with the notifications and what users are willing to subscribe to.

### Solution brainstorm

Introduce an activity hub that will collect all the notifications in the UI, while keeping the option to receive them by e-mail (and possibly other channels in the future).

The notifications should track state as well, so, for example, a request to add a new language would no longer be actionable once the language is added to the component.

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

It should cover at least following use cases:

* https://github.com/WeblateOrg/weblate/issues/5533
* https://github.com/WeblateOrg/weblate/issues/9708
* https://github.com/WeblateOrg/weblate/issues/6569

Also it should allow more effective way configuring notifications to be able to add more notification types without overwhelming user interface:

* https://github.com/WeblateOrg/weblate/issues/4865
* https://github.com/WeblateOrg/weblate/issues/12622
* https://github.com/WeblateOrg/weblate/issues/19807
* https://github.com/WeblateOrg/weblate/issues/5155
* https://github.com/WeblateOrg/weblate/issues/18437
* https://github.com/WeblateOrg/weblate/issues/18419

Review the linked use-case issues 5533, 9708, 6569, 4865, 12622, 19807, 5155, 18437, and 18419 to understand the notification requirements. The work is complete when an in-UI activity hub collects notifications, preserves email delivery, tracks notification state, and supports clearer notification configuration.

Written by the indexing model from the issue text.